Extruders having a coaxial screw-press with oppositely oriented external screw threads to feed different plastic substances to the nozzle, the outer screw being rotatable while the inner screw is stationary are described in Austrian Pat. No. 198,501 and a derivative of the same is described in U.S. Pat. No. 3,999,921 where the nozzle-side end of the outer screw has attached thereto a mixing element with vane type mixing tools, the tools sweeping the transport space of the outer screw as well as that of the inner screw. Thereby it is said to be possible to produce an extrusion with marbled texture evenly distributed over the cross section, which subsequently can be further processed, for example, to form marbled cakes of soap. The rotating mixing tools convey particles from the substance stream transported by the inner screw into the substance stream transported by the outer screw, as well as conversely conveying particles from the periphery inwardly. It is thereby prevented that areas without marbled texture can form. The result of this partial mixing of substance streams, while good in itself, depends on a plurality of other factors besides the technical design of the mixing element (form and number of the mixing tools). Thus, at a given speed of rotation of the outer screw, different temperatures and varying consistencies of the substances to be processed and also different pressures in the mixing zone lead to varying marbled patterns, giving the products made from the extrusion, such as compacted cakes of soap, a varying appearance or texture. Besides, the mixing tools, of relatively compact design, lead to an undesired pressure build-up, with the result that the output of the extruder is reduced.